On the first 3 quizzes of his geometry class, William got an average score of 84. What does he need on the next quiz to have an overall average of 87?
Explanation: Let his score on the next quiz be $x$ The sum of all of his scores is then $3 \cdot 84 + x$ The same sum must also be equal to $4 \cdot 87$ Solve: $x = 4 \cdot 87 - 3 \cdot 84 = 96$.